Galvanic cells, or Voltaic cells derive electrical energy from chemical reactions taking place within the cell. They generally consist of two different metals and an electrolyte. When the dissimilar metals come in contact with a common electrolyte, a potential difference is created between the metals. Once an electron path is provided, external to the cell itself, electrons flow from the anode to the cathode. Electrons flow from the anode to the cathode, depleting atoms of electrons, causing the remaining atoms to become ions.
These cells are more generally referred to within the public domain as batteries and are more predominantly used as a means of storing electrical energy.
However, some applications of these cells, like certain timing systems, temperature indicators and visual indicators, capitalize on other attributes inherent to these cells. One particular attribute of interest is the transformation of molecules within the anode from atom to ion and the subsequent change in optical properties. The optical properties of the anode change from opaque to transparent as atoms become ions.
The change in optical properties is relied upon within certain timing systems, temperature indicators and visual indicators, also referred to as time dependent color changing labels. Within these applications anode material consists of a thin metal film which has been deposited by evaporation or sputter or similar technique and configured on the same plane or nearly the same plane as a cathode such that when an electrolyte is introduced, anode atoms begin to deplete themselves of electrons and transform into ions, beginning at a point closest to the cathode. As depletion continues an ion rich transparent region begins to expand in a direction away from the cathode.
As the optical properties of the anode change from opaque to transparent backgrounds that used to lay hidden become visible. The expansion of the transparent region reveals various colors, text and/or patterns which have been printed just behind the anode. Progression of the transparent region indicates that increasing intervals of time have expired based on the appearance of colors text and/or patterns. This describes the general operation of some expiration indicators or color changing labels.